Traditionally, in the pre-press industry, printing members to be used in a printing press are recorded by a contact process from a photographic film produced by a laser plotter, the film including a half tone image representing the original image to be subsequently reproduced in the printing press.
In the last decade, it has been realized that the digital representation of an image to be reproduced in a printing press may be recorded directly on a printing member, thus obviating the use of films. This process is usually termed computer to plate or computer to press, both usually referred to as CTP.
In computer to plate process, the printing member is produced directly by a plate setter, such as the Raystar, commercially available from Scitex Corporation Ltd. of Herzlia, Israel. In the computer to press process, the printing member is produced on a digital press, in particular in a digital lithographic press, such as the Quick Master DI46-4, commercially available from Heidelberg Druckmachinen of Germany. In this process, the image is recorded on the printing member on-press, i.e. while the printing plate is mounted on the printing press.
Printing members suitable for CTP include visible light sensitive silver halide based printing members, ablative printing members and photopolymer printing members. Examples for silver halide based printing members include the SETPRINT HN, commercially available from Agfa Gaevert of Belgium or the SDP-FH100/175, commercially available from Mitsubishi of Japan. An example of an ablative printing member include the Pearl plate, commercially available from Presstek Inc. of New Hampshire, U.S. An example of a photopolymer printing member includes the 3M Viking, commercially available from Minnesota Mining and Manufacturing company (3M) of the U.S.
Prior art CTP printing members require a process other than the recording of the image representing the image to be reproduced by the printing press thereon. In the Pearl and 3M Viking printing members the process is post imaging, i.e., for the Pearl Plate the ablated portions, which are parts physically removed from the printing member so as to form the image thereon, have to be removed before actual printing. Similarly, for the 3M Viking plate a washing process is required.
In the silver halide based printing members further processing is required. Since these printing members are visible light sensitive, they are first imaged in a dark, visible light free environment, by a visible light exposure beam so as to record the image to be reproduced thereon as indicated in step 2 of FIG. 1 to which reference is now made. The exposure beam, usually a modulated red light laser beam in the range of 620-670 nanometers records a pattern representing the image to be printed on the printing member. Then as indicated by 4, the printing member is processed by dedicated chemistry to form the processed printing member ready for mounting on a printing press.
As the printing member is sensitive to visible light, the processing step 4 is also carried in a dark, visible light free environment.
The processing step 4 usually includes the use of an activator and a stabilizer, such as the commercially available Mitsubishi Silver Digiplate System including the SLM-AC activator and the SLM-ST stabilizer. The use of the activator and the stabilizer are followed by a washing step, usually by water, and subsequent drying.
Thus, a major drawback of prior art CTP printing members is that they require processing of some sort at the user's end other than the recording of the pattern representing the image to be reproduced thereon.
In particular, a major drawback of light sensitive silver halide based prior art printing members is that the entire process including exposure and chemical processing must be carried in a dark, visible light free environment as indicated by the dashed lines referenced 6.
Prior art silver halide based printing members, such as printing member 20 illustrated in FIG. 2 include a support base layer 22, such as a polyester, paper or metal base layer and a silver halide emulsion layer 24, such as a gelatine based layer having silver halide therein, the layer 24 being coated on base layer 22. After exposure in a plate setter, the printing member is chemically processed as described above. The resulting reduced silver areas in the exposed portions are oleophilic whereas the background is hydrophilic.